Fix intermittent failure in test_POST_but_expired
Fix intermittent failure in test_POST_but_expired

It was rare (I saw it once in 8000 runs), but occassionally the clock would
roll over and cause us to PUT an X-Delete-At that matched the current time.

Change-Id: I10d3b9fcbefff3c415a92fa284a1ea1eda458581
Closes-Bug: #1597520

clayg/swift,matthewoliver/swift,matthewoliver/swift,nadeemsyed/swift,tipabu/swift,swiftstack/swift,smerritt/swift,smerritt/swift,openstack/swift,nadeemsyed/swift,clayg/swift,clayg/swift,matthewoliver/swift,psachin/swift,psachin/swift,smerritt/swift,swiftstack/swift,openstack/swift,openstack/swift,nadeemsyed/swift,tipabu/swift,openstack/swift,nadeemsyed/swift,tipabu/swift,clayg/swift,psachin/swift,smerritt/swift,swiftstack/swift,psachin/swift,tipabu/swift,matthewoliver/swift